By Kelly M. Pyrek
One of the basic infection control principles is the chain of infection. Transmission of infection in a hospital requires at least three elements: a source of infecting microorganisms, a susceptible host and a means of transmission for bacteria and viruses.1
"An example of the most simple chain of infection is an infected patient cared for by a healthcare worker (HCW) who doesn't wash his or her hands before caring for another patient," says Richard Wenzel, MD, MSc, of the Department of Internal Medicine of Virginia Commonwealth University in Richmond, Va. Wenzel adds that contaminated hands are one of the most likely means of transmission of bacteria in hospitals.
"When you talk about the chain of infection, you must acknowledge Ignaz Semmelweis, who in the mid-1800s, noted how bacteria travel from caregiver to patient." In a chapter on handwashing for A Guide to Infection Control in the Hospital, Wenzel describes how, as an obstetrician, Semmelweiss noticed the practice of physicians and medical students examining women who died of puerperal sepsis (later linked to infection by Streptococcus pyogenes) and then going directly to the wards where they examined women in labor. Wenzel writes, "Semmelweis noted that on wards where midwives delivered babies, few mothers died of puerperal sepsis. He knew that midwives did not witness autopsies. Semmelweis reasoned that something was carried from the autopsy room to the wards on the hands of physicians and students. He introduced a simple handwashing regimen and rates of death due to puerperal sepsis fell."2
"Obviously hands are important agents in the transmission of infection and a number of studies demonstrate this," Wenzel says. "In the 1970s, Katherine Sprunt's work with newborns showed that if you washed your hands after changing a baby's diaper, you get rid of the coliforms, the transient gram-negative rods. Then it turns out that gram-positive bacteria are not so easily washed off the hands. In the 1980s, Ojajarvi of Finland showed us that medicated soaps worked better than plain soap and water for gram-positive bacteria, particularly methicillin-resistant Staph aureus (MRSA). It still stands to reason, however, that a careful 10- to 15-second handwash with any soap and water is effective in reducing the transmission of bacteria."
Wenzel continues, "In the 1960s, one study of newborn units looked at how Staph aureus was transmitted. Physicians, nurses and babies were cohorted in one area while another colonized group was cohorted where the only transmission of bacteria would have occurred by an airborne route. Nobody crossed the magic line between the two groups, and at the end of the study it was shown that about 15 percent of the transmission of Staph aureus could have been by the airborne route. There was no other explanation, so at least 85 percent of the transmission was via hands. The concept of the chain of infection started with initial studies on the importance of handwashing and was confirmed in subsequent decades."
The specific links in the chain of infection are: reservoir, infectious agent, susceptible host, portal of entry, mode of transmission and portal of exit.3 Each link must be present and in sequential order for an infection to occur.
Getting to the Source
Human sources of microorganisms are healthcare workers (HCWs), patients and visitors, any of whom may be individuals in the incubation period of a disease, those who already have a disease, or those who are considered to be chronic carriers of an infectious agent. Other sources of bacteria are the patient's endogenous (produced or originating from a cell or organism) flora and inanimate objects that have become contaminated (see next month's issue of Infection Control Today for an article on fomites).
"We know from various studies that for most people, it takes about 100,000 organisms to transmit Staph aureus to an open wound on the body, such as an abrasion or surgical site," Wenzel says. "Most infections are from endogenous sources. For healthy people who go into surgery and later develop an infection, it's 50 percent colonized Staph aureus and 50 percent exogenous bacteria. It's complicated because sometimes patients come in to the hospital and they become colonized; the organism is then endogenous and it stays that way for a week or two and then they become infected with the new organism they picked up. Is the bacterial infection endogenous or exogenous [originating outside]? The terms have to be defined carefully. For example, almost all bloodstream infections due to Candida are from the patient's endogenous flora, but many patients with Candida might have picked it up from someone else in the hospital."
A Welcoming Host
Certain individuals are immune to infection or are able to resist colonization by an infectious agent while others develop a clinical disease. Still others are able to establish a commensal relationship with bacteria and become asymptomatic carriers. With such a wide spectrum of host resistance, factors that can make or break infectivity include: age; underlying diseases; treatments with antimicrobials or other immunosuppressive agents; irradiation; and breaks in the first line of defense mechanisms caused by surgical operations, anesthesia and indwelling catheters that may render patients more susceptible to infection.4 According to guidelines from the Association for Professionals in Infection Control and Epidemiology (APIC), subclinical cases and carriers present risks of transmission to susceptible hosts in hospitals for the following reasons:5
Despite the "invisibility" of these subclinical cases and carriers, Wenzel says hospitals can take certain steps to protect its healthcare personnel and its patients. "The best precaution is for a hospital to have a culture where HCWs who are sick with a cold or the flu know that they won't have to work while they are ill," he says. "Obviously there is enormous pressure to keep working, even though you might be sick. People are afraid of losing their jobs or looking weak while they're out sick and their colleagues are working overtime. Hospitals need to tell sick employees, 'We don't want you here if you are sick and your patients definitely don't want you here because your viral infection has the potential to transmit large droplets contaminated with infectious agents.' Plus there has to be the culture where strict handwashing policies are adhered to."
Hitching a Ride
While there are five main routes of transmission of bacteria -- contact, droplet, airborne, common vehicle and vectorborne -- the first three routes are most critical to the discussion of nosocomial infections.6
Two modes of contact transmission exist, direct-contact transmission and indirect-contact transmission. Direct-contact transmission, as its name implies, consist of direct body surface to body surface contact and physical transfer of bacteria between a susceptible host and an infected or colonized individual. Examples of this kind of contact include bathing or turning a patient. Indirect-contact transmission involves contact of a susceptible host with a contaminated object such as medical instruments, dressings, gloves that are not changed between patients or unwashed hands.
Wenzel points to a classic example of a chain of infection demonstrating contact transmission.
"In an ICU, where people are crowded together and HCWs are frantically running from patient to patient, handwashing might not be adhered to assiduously enough. HCWs going from a trach or a wound site to an IV catheter to a Foley without washing their hands can trigger infection among many patients," Wenzel says. "Other scenarios are having a carrier with a cold working in an operating room and using inadequate barrier protection; large droplets [from a cough or sneeze] could land on the wound. Theoretically, that could happen in a critical care unit too, where nobody wears a mask most of the time."
Infectious droplets are generated during coughing, sneezing, talking and through procedures such as bronchoscopy and suctioning. Transmission occurs when these droplets are propelled a short distance through the air and deposited on a host's mouth, nasal mucosa or conjunctivae. Droplet transmission is not to be confused with airborne transmission because droplets do not remain suspended in the air and special air handling and ventilation is not required.
This kind of transmission consist of dust particles containing bacteria or airborne droplet nuclei (small-particle residue 5 mm or smaller in size) of evaporated droplets containing microorganisms that are suspended in the air for longer durations of time. Microorganisms transmitted in this manner can be inhaled by a susceptible host, so special air handling is indicated to prevent the spread of infection. Smallpox and Mycobacterium tuberculosis are a few examples of bacteria spread by airborne transmission.
Infection control experts agree that following standard precautions, isolation guidelines when required and engaging in proper handwashing is essential to breaking the chain of infection. Following aseptic technique is especially critical when caring for patients with hardy microorganisms such as MRSA. A HCW can break the chain in the following way: